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In category theory and its applications to other branches of mathematics, **kernels** are a generalization of the kernels of group homomorphisms, the kernels of module homomorphisms and certain other kernels from algebra. Intuitively, the kernel of the morphism *f* : *X* → *Y* is the "most general" morphism *k* : *K* → *X* that yields zero when composed with (followed by) *f*.

- Definition
- Examples
- Relation to other categorical concepts
- Relationship to algebraic kernels
- Sources

Note that kernel pairs and difference kernels (also known as binary equalisers) sometimes go by the name "kernel"; while related, these aren't quite the same thing and are not discussed in this article.

Let **C** be a category. In order to define a kernel in the general category-theoretical sense, **C** needs to have zero morphisms. In that case, if *f* : *X* → *Y* is an arbitrary morphism in **C**, then a kernel of *f* is an equaliser of *f* and the zero morphism from *X* to *Y*. In symbols:

- ker(
*f*) = eq(*f*, 0_{XY})

To be more explicit, the following universal property can be used. A kernel of *f* is an object *K* together with a morphism *k* : *K* → *X* such that:

*f*∘*k*is the zero morphism from*K*to*Y*;

- Given any morphism
*k*′ :*K*′ →*X*such that*f*∘*k*′ is the zero morphism, there is a unique morphism*u*:*K*′ →*K*such that*k*∘*u*=*k′*.

Note that in many concrete contexts, one would refer to the object *K* as the "kernel", rather than the morphism *k*. In those situations, *K* would be a subset of *X*, and that would be sufficient to reconstruct *k* as an inclusion map; in the nonconcrete case, in contrast, we need the morphism *k* to describe *how**K* is to be interpreted as a subobject of *X*. In any case, one can show that *k* is always a monomorphism (in the categorical sense). One may prefer to think of the kernel as the pair (*K*, *k*) rather than as simply *K* or *k* alone.

Not every morphism needs to have a kernel, but if it does, then all its kernels are isomorphic in a strong sense: if *k* : *K* → *X* and *ℓ* : *L* → *X* are kernels of *f* : *X* → *Y*, then there exists a unique isomorphism φ : *K* → *L* such that *ℓ*∘φ = *k*.

Kernels are familiar in many categories from abstract algebra, such as the category of groups or the category of (left) modules over a fixed ring (including vector spaces over a fixed field). To be explicit, if *f* : *X* → *Y* is a homomorphism in one of these categories, and *K* is its kernel in the usual algebraic sense, then *K* is a subalgebra of *X* and the inclusion homomorphism from *K* to *X* is a kernel in the categorical sense.

Note that in the category of monoids, category-theoretic kernels exist just as for groups, but these kernels don't carry sufficient information for algebraic purposes. Therefore, the notion of kernel studied in monoid theory is slightly different (see #Relationship to algebraic kernels below).

In the category of unital rings, there are no kernels in the category-theoretic sense; indeed, this category does not even have zero morphisms. Nevertheless, there is still a notion of kernel studied in ring theory that corresponds to kernels in the category of non-unital rings.

In the category of pointed topological spaces, if *f* : *X* → *Y* is a continuous pointed map, then the preimage of the distinguished point, *K*, is a subspace of *X*. The inclusion map of *K* into *X* is the categorical kernel of *f*.

The dual concept to that of kernel is that of cokernel. That is, the kernel of a morphism is its cokernel in the opposite category, and vice versa.

As mentioned above, a kernel is a type of binary equaliser, or difference kernel. Conversely, in a preadditive category, every binary equaliser can be constructed as a kernel. To be specific, the equaliser of the morphisms *f* and *g* is the kernel of the difference *g*−*f*. In symbols:

- eq (
*f*,*g*) = ker (*g*−*f*).

It is because of this fact that binary equalisers are called "difference kernels", even in non-preadditive categories where morphisms cannot be subtracted.

Every kernel, like any other equaliser, is a monomorphism. Conversely, a monomorphism is called * normal * if it is the kernel of some morphism. A category is called *normal* if every monomorphism is normal.

Abelian categories, in particular, are always normal. In this situation, the kernel of the cokernel of any morphism (which always exists in an abelian category) turns out to be the image of that morphism; in symbols:

- im
*f*= ker coker*f*(in an abelian category)

When *m* is a monomorphism, it must be its own image; thus, not only are abelian categories normal, so that every monomorphism is a kernel, but we also know *which* morphism the monomorphism is a kernel of, to wit, its cokernel. In symbols:

*m*= ker (coker*m*) (for monomorphisms in an abelian category)

Universal algebra defines a notion of kernel for homomorphisms between two algebraic structures of the same kind. This concept of kernel measures how far the given homomorphism is from being injective. There is some overlap between this algebraic notion and the categorical notion of kernel since both generalize the situation of groups and modules mentioned above. In general, however, the universal-algebraic notion of kernel is more like the category-theoretic concept of kernel pair. In particular, kernel pairs can be used to interpret kernels in monoid theory or ring theory in category-theoretic terms.

- Awodey, Steve (2010) [2006].
*Category Theory*(PDF). Oxford Logic Guides.**49**(2nd ed.). Oxford University Press. ISBN 978-0-19-923718-0. - Kernel in
*nLab*

In mathematics, given two groups, and, a **group homomorphism** from to is a function *h* : *G* → *H* such that for all *u* and *v* in *G* it holds that

In algebra, a **homomorphism** is a structure-preserving map between two algebraic structures of the same type. The word *homomorphism* comes from the ancient Greek language: *ὁμός (homos)* meaning "same" and *μορφή (morphe)* meaning "form" or "shape". However, the word was apparently introduced to mathematics due to a (mis)translation of German *ähnlich* meaning "similar" to *ὁμός* meaning "same". The term "homomorphism" appeared as early as 1892 ; it was attributed to the German mathematician Felix Klein (1849–1925).

In mathematics, an **abelian category** is a category in which morphisms and objects can be added and in which kernels and cokernels exist and have desirable properties. The motivating prototype example of an abelian category is the category of abelian groups, **Ab**. The theory originated in an effort to unify several cohomology theories by Alexander Grothendieck and independently in the slightly earlier work of David Buchsbaum. Abelian categories are very *stable* categories; for example they are regular and they satisfy the snake lemma. The class of abelian categories is closed under several categorical constructions, for example, the category of chain complexes of an abelian category, or the category of functors from a small category to an abelian category are abelian as well. These stability properties make them inevitable in homological algebra and beyond; the theory has major applications in algebraic geometry, cohomology and pure category theory. Abelian categories are named after Niels Henrik Abel.

In algebra, the **kernel** of a homomorphism is generally the inverse image of 0. An important special case is the kernel of a linear map. The kernel of a matrix, also called the *null space*, is the kernel of the linear map defined by the matrix.

In mathematics, specifically abstract algebra, the **isomorphism theorems** are three theorems that describe the relationship between quotients, homomorphisms, and subobjects. Versions of the theorems exist for groups, rings, vector spaces, modules, Lie algebras, and various other algebraic structures. In universal algebra, the isomorphism theorems can be generalized to the context of algebras and congruences.

In mathematics, a **category** is a collection of "objects" that are linked by "arrows". A category has two basic properties: the ability to compose the arrows associatively and the existence of an identity arrow for each object. A simple example is the category of sets, whose objects are sets and whose arrows are functions.

In mathematics, specifically in category theory, a **preadditive category** is another name for an **Ab-category**, i.e., a category that is enriched over the category of abelian groups, **Ab**. That is, an **Ab-category****C** is a category such that every hom-set Hom(*A*,*B*) in **C** has the structure of an abelian group, and composition of morphisms is bilinear, in the sense that composition of morphisms distributes over the group operation. In formulas:

In category theory, an **epimorphism** is a morphism *f* : *X* → *Y* that is right-cancellative in the sense that, for all objects *Z* and all morphisms *g*_{1}, *g*_{2}: *Y* → *Z*,

In mathematics, specifically in category theory, an **additive category** is a preadditive category **C** admitting all finitary biproducts.

In mathematics, specifically in category theory, a **pre-abelian category** is an additive category that has all kernels and cokernels.

The **snake lemma** is a tool used in mathematics, particularly homological algebra, to construct long exact sequences. The snake lemma is valid in every abelian category and is a crucial tool in homological algebra and its applications, for instance in algebraic topology. Homomorphisms constructed with its help are generally called *connecting homomorphisms*.

An **exact sequence** is a concept in mathematics, especially in group theory, ring and module theory, homological algebra, as well as in differential geometry. An exact sequence is a sequence, either finite or infinite, of objects and morphisms between them such that the image of one morphism equals the kernel of the next.

**Homological algebra** is the branch of mathematics that studies homology in a general algebraic setting. It is a relatively young discipline, whose origins can be traced to investigations in combinatorial topology and abstract algebra at the end of the 19th century, chiefly by Henri Poincaré and David Hilbert.

In mathematics, the **cokernel** of a linear mapping of vector spaces *f* : *X* → *Y* is the quotient space *Y* / im(*f*) of the codomain of *f* by the image of *f*. The dimension of the cokernel is called the *corank* of *f*.

In mathematics, an **equalizer** is a set of arguments where two or more functions have equal values. An equalizer is the solution set of an equation. In certain contexts, a **difference kernel** is the equalizer of exactly two functions.

The following outline is provided as an overview of and guide to category theory, the area of study in mathematics that examines in an abstract way the properties of particular mathematical concepts, by formalising them as collections of *objects* and *arrows*, where these collections satisfy certain basic conditions. Many significant areas of mathematics can be formalised as categories, and the use of category theory allows many intricate and subtle mathematical results in these fields to be stated, and proved, in a much simpler way than without the use of categories.

In category theory, a **coequalizer** is a generalization of a quotient by an equivalence relation to objects in an arbitrary category. It is the categorical construction dual to the equalizer.

In mathematics, the category **Ab** has the abelian groups as objects and group homomorphisms as morphisms. This is the prototype of an abelian category: indeed, every small abelian category can be embedded in **Ab**.

In mathematics, the category **Grp** has the class of all groups for objects and group homomorphisms for morphisms. As such, it is a concrete category. The study of this category is known as group theory.

In mathematics, the **category of rings**, denoted by **Ring**, is the category whose objects are rings and whose morphisms are ring homomorphisms. Like many categories in mathematics, the category of rings is large, meaning that the class of all rings is proper.

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